The present invention relates to an analog-to-digital (hereinafter often simply referred to as A/D) converter which converts an analog voltage signal into a corresponding digital signal. More specifically, the invention relates to an A/D converter of the over-sampling type adapted to being realized in a semiconductor integrated circuit, and to a sampling pulse-forming circuit most adapted to the A/D converter. The invention is further related to a current switching circuit whose output current value is controlled by a digital signal which consists of a plurality of bits, and to an A/D converter in which the current switching circuit works as a local digital-to-analog (hereinafter often simply referred to as D/A) converter circuit, and particularly to an over-sampling A/D converter of the type of current integration capable of realizing high-precision conversion characteristics without requiring high precision in the current value.
One of the systems of the A/D converter can be represented by an over-sampling system which uses a sampling frequency that is several tens to several hundreds of times as great as the frequency band of analog input signals. The over-sampling A/D converter can be classified into several types depending on the circuit constitution, such as one of the delta-sigma (.DELTA..SIGMA.) type disclosed in, for example, Hauser et al, "MOS Falter . . . " 1985 IEEE International Solid-State Circuits Conference, pp. 80-81, February, 1985 and one of the interpolation type disclosed in for example, Yamakido et al, "A Voiceband 15b Interpolative Converter Chip Set" 1986 IEEE International Solid-State Circuits Conference pp. 180-181, February, 1986. The A/D converters of the over-sampling system require high-speed operation but permit the elements in the analog circuitry to have a greatly decreased level of accuracy compared with those of other non-over-sampling system devices making it therefore possible to relatively easily realize conversion characteristics maintaining high accuracy relying upon a semiconductor integrated circuit. Moreover, a pre-filter that is usually provided in the preceding stage of all A/D converters can be realized in a small size, maintaining loose element accuracy in order to prevent reflection in the band caused by the sampling of high-band noise components included in the input signals.
In many conventional over-sampling A/D converters, however, an operational amplifier has been used the integrated circuit portion for integrating error voltages between the input analog signal and the analog reference signal that is fed back and a limitation is imposed on reducing the electric power consumed by the A/D converter since the operational amplifier must be operated at high speeds. Conversely, a limitation is imposed on the speed of the operational amplifier even if it is attempted to increase the speed by increasing the consumption of the electric power, and it becomes difficult, too, to obtain further increased conversion precision even if the sampling frequency is increased.
The over-sampling A/D converters have heretofore used a power-source voltage which is 5 volts or greater. It was, however, found through study by the present inventors that when it is attempted to adapt the over-sampling A/D converter to small portable communication terminal equipment such as a pager (or generally called pocket bell), a cordless telephone set and a mobile telephone set that have been vigorously developed in recent years, the operation must be sustained on small cells, i.e., the operation must be sustained on a low-voltage power source. Even in this case, the operational amplifier must be operated at high speeds making it further difficult to accomplish the conversion maintaining required accuracy.
In order to solve the above-mentioned problems, furthermore, there has been proposed a circuit system in which no operational amplifier is required for the analog integration circuit portion as is disclosed in Koch et al, "A 12-bit Sigma-Delta Analog-to-digital Converter with a 15-MHz Clock Rate" IEEE, JOURNAL OF SOLID-STATE CIRCUITS, SC-21, December, 1986, pp. 1003-1010. According to this circuit system, the input analog voltage signal is converted into a current signal using a voltage-to-current converter circuit and, at the same time, a local D/A converter circuit is constituted by a current switching circuit of one bit, in order to electrically charge or integrate a differential current between the input signal and the feedback signal using a capacitor whose one end is grounded to a dc potential as an analog integration circuit. Therefore, the above circuit system is of the .DELTA..SIGMA. type.
The voltage-to-current converter circuit for the input signals requires an operational amplifier to maintain linear precision to some extent. The operational amplifier needs to maintain the gain to some extent for the frequency band of the desired input signals. Further, the load that must be driven by the operational amplifier consists solely of a gate electrode of a transistor such as the MOS transistor connected to the current source, in order to decrease the consumption of electric power.
General technical discussions related to the A/D and D/A converters have been published in, for example, Nikkei Journal of Electronics, No. 447, 1988 pp. 165-175, the same journal, No. 452 1988 pp. 277-285, the same journal, No. 493, 1988, pp. 211-221, the same journal, No. 454, 1988, pp. 277-285, ICASSP85, Proceedings, 36. 7. 1-36. 7. 4 (1985), pp. 1400-1403, and Japanese Patent Laid-Open No. 65626/1986.